1. Field
The present disclosure relates generally to methods of selectively separating an alcohol from an aqueous mixture using membranes derived from copolymer compositions; more specifically it relates to methods of selectively separating ethanol from an aqueous mixture using membranes derived from polystyrene-polybutadiene-polystyrene block copolymers having a cylindrical morphology or from copolymers of ring-opening metathesis polymerization, where one of the cycloalkene monomers is substituted with one or more polydialkylsiloxane groups.
2. Related Art
The use of renewable feedstocks for conversion to transportation fuels is growing rapidly. As corn grain use plateaus, the next large source of feedstock is lignocellulosic biomass. A difficulty with this source is that the fermentable C6 sugars that may be derived from its cellulose content (˜44%) are less concentrated than those that are derived from high starch (˜66%) grains. Significant amounts of hemicellulose (˜30%) drive up the viscosity in the fermentor. In addition, fermentation inhibitors will be present, though this depends on the pretreatment hydrolysis method. For these reasons, lignocellulose-based fermentations are run more dilute than high starch grain fermentations, and the alcohol product is necessarily significantly less concentrated. This is a problem when distillation is used as the alcohol recovery method. In modern corn dry-grind ethanol plants with significant heat integration, the energy to distill the 10-13 wt % ethanol feed and dehydrate it to fuel specification is equivalent to ˜20% of its lower heating value. As feed concentration drops (i.e. for lignocellulosic feedstocks), energy use (and cost) rises exponentially. Butanol produced by fermentation is even more dilute, as the product is a much stronger inhibitor to the fermenting microorganisms than ethanol. Typical butanol fermentations result in 1-2 wt % butanol in the broth.
A potentially lower-energy alternative to distillation for recovery of ethanol or other alcohols is membrane permeation. The process may be vapor permeation (vapor feed/vapor permeate) or pervaporation (liquid feed/vapor permeate). Pervaporation is already in commercial use for dehydration of ethanol (4-20 wt % water content) and other solvents by permeation through water-selective membranes. Permeation of the minor component is preferred, to minimize the membrane area required. Currently alcohol-selective membrane materials have not been found that perform well enough to compete commercially with distillation. The reason is that the majority of membranes are water-selective. Permeation selectivity is the product of solubility selectivity and diffusivity selectivity. Water, being a smaller molecule than the alcohols, diffuses faster. Water-selective membranes such as polyvinyl alcohol (PVA) have pervaporation process separation factors above 200, compared to ˜9 for polydimethylsiloxane (PDMS) permeating ethanol. Besides the potential for energy savings, particularly for lower concentration feeds, membrane permeation lends itself to continuous removal of the product as it is formed. This may result in higher fermentor productivity, and higher yields. This is especially important for the production of butanol, where inhibition occurs well-before available sugar is consumed.
A variety of alcohol-selective membrane materials have been investigated including individual polymers, polymer blends, block copolymers, interpenetrating network polymer blends, surface-modified polymers, hydrophobic zeolite crystals intergrown to form a film, polymer-coated zeolite films, mixed matrix membranes incorporating zeolite particles into a polymer matrix, and liquid membranes. The membranes that have resulted from these approaches have their individual strengths and weaknesses, but none have resulted in a commercial application for alcohol recovery. Silicone rubber polydimethylsiloxane (PDMS) has demonstrated some success in alcohol pervaporations, and crosslinking of PDMS is typically needed to enhance the mechanical stability of the membranes. Crosslinked PDMS membranes exhibit an ethanol/water selectivity of 9. Additionally, block copolymers containing a PDMS block have been explored. For example, graft copolymers consisting of a poly(1-trimethylsilylpropyne) (PTMSP) backbone with PDMS grafts and of a poly(1-phenylpropyne) (PPP) backbone with PDMS grafts exhibited high ethanol/water selectivities of 28 and 40, respectively, but were shown to give unstable performance with flux and selectivity declining with time. Other block copolymers containing PDMS, such as polymethylmethacrylate (PMMA)-block (or graft)-PDMS, polystyrene (PS)-graft-PDMS, and polysulfone-graft-PDMS, have shown ethanol separation behavior similar to crosslinked PDMS membranes. Due to the low mechanical stability of the matrix phase (e.g. PS, PMMA), the resultant copolymer membranes were brittle and difficult to handle. Moreover, the effects of the PDMS size and morphology have not been explicitly addressed in those examples. Thus, there is a need for better-performing alcohol-selective membranes.